Semiconductor switching string

ABSTRACT

A semiconductor switching string includes a plurality of series-connected semiconductor switching assemblies, each having a main semiconductor switching element that includes first and second connection terminals. The main semiconductor switching element also has an auxiliary semiconductor switching element electrically connected between the first and second connection terminals. Each semiconductor switching assembly also includes a control unit configured to switch on a respective auxiliary semiconductor switching element to selectively create an alternative current path between the first and second connection terminals whereby current is diverted to flow through the alternative current path to reduce the voltage across the corresponding main semiconductor switching element. The or each control unit is further configured to switch on the auxiliary semiconductor switching element when the voltage across the corresponding main semiconductor switching element differs from a voltage reference derived from the voltage across all of the main semiconductor switching elements.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a National Stage of International ApplicationNo. PCT/EP2014/062049, filed Jun. 10, 2014, which claims priority toEuropean Application No. 13172036, filed Jun. 14, 2013, which isincorporated herein by reference in its entirety.

This invention relates to a semiconductor switching string for use in ahigh voltage direct current (HVDC) power converter.

In power transmission networks alternating current (AC) power istypically converted to direct current (DC) power for transmission viaoverhead lines and/or under-sea cables. This conversion removes the needto compensate for the AC capacitive load effects imposed by thetransmission line or cable and reduces the cost per kilometre of thelines and/or cables, and thus becomes cost-effective when power needs tobe transmitted over a long distance.

HVDC power converters are used to convert AC power to DC power.Semiconductor switching elements, such as thyristors, are a keycomponent of HVDC power converters, and act as controlled rectifiers toconvert AC power to DC power and vice versa.

While such semiconductor switching elements have very high breakdownvoltages and are capable of carrying high current loads, evensemiconductor switching elements from the same batch exhibit differentperformance characteristics. This creates difficulties in the operationof, e.g. a HVDC power converter in which the semiconductor switchingelements are incorporated.

In addition, many semiconductor switching elements have inherentlimitations in their performance which require the inclusion of large,heavy and difficult-to-design remedial components within, e.g. a HVDCpower converter, to compensate for these shortcomings.

There is, therefore, a need for an improved semiconductor switchingassembly which obviates one or more of the difficulties outlined above.

According to a first aspect of the invention there is provided asemiconductor switching string, for use in a HVDC power converter,comprising:

-   -   a plurality of series-connected semiconductor switching        assemblies, each semiconductor switching assembly having a main        semiconductor switching element including first and second        connection terminals between which current flows from the first        terminal to the second terminal when the main semiconductor        switching element is switched on, the main semiconductor        switching element having an auxiliary semiconductor switching        element electrically connected between the first and second        connection terminals thereof; and    -   a control unit operatively connected with each auxiliary        semiconductor switching element, the or each control unit being        configured to switch on a respective auxiliary semiconductor        switching element to selectively create an alternative current        path between the first and second connection terminals        associated therewith whereby current is diverted to flow through        the alternative current path to reduce the voltage across the        corresponding main semiconductor switching element, and the or        each control unit being further configured to switch on the said        respective auxiliary semiconductor switching element when the        voltage across the corresponding main semiconductor switching        element differs from a voltage reference derived from the        voltage across all of the main semiconductor switching elements        in the semiconductor switching string.

The inclusion of a control unit which is configured to switch on arespective auxiliary semiconductor switching element to selectivelycreate an alternative current path between the first and secondconnection terminals of a corresponding main semiconductor switchingelement, whereby current is diverted through the alternative currentpath to reduce the voltage across the corresponding main semiconductorswitching element, allows the semiconductor switching string of theinvention to compensate for a variation in the turn-off performancecharacteristic of the various main semiconductor switching elements inthe string of series-connected main semiconductor switching elements. Assuch the semiconductor switching string of the invention permits the mixand match of semiconductor switching elements, e.g. thyristors, from notjust different batches but from different suppliers. Furthermore, theswitching string of the invention drastically reduces the size of anassociated remedial component, e.g. a damping circuit, that is otherwiserequired to compensate for the aforementioned variation in the turn-offperformance characteristic of a series of semiconductor switchingelements.

In addition, having a control unit which is additionally configured toswitch on a respective auxiliary semiconductor switching element whenthe voltage across the corresponding main semiconductor switchingelement differs from a voltage reference provides a degree of feedbackcontrol via which the aforementioned compensation for variations in theturn-off characteristics of the various main semiconductor switchingelements can be carried out automatically.

Preferably the voltage reference is equivalent to the average voltageacross all of the main semiconductor switching elements in thesemiconductor switching string.

Such a reference voltage can be readily computed, e.g. by a higher levelcontroller which receives from the or each control unit a localmeasurement of the voltage across the corresponding main semiconductorswitching element.

Optionally the or each control unit is configured to switch on a saidrespective auxiliary semiconductor switching element when the voltageacross the corresponding main semiconductor switching element is greaterthan the voltage reference.

A control unit so configured has the effect of reducing the voltageacross the or each main semiconductor switching element with a voltageabove the voltage reference, i.e. above the average voltage across allof the main semiconductor switching elements, such that the averagevoltage across the remaining main semiconductor switching elementsincreases and in due course the voltage across all of the mainsemiconductor switching elements becomes essentially equally balancedacross each individual main semiconductor switching element.

The or each control unit may be configured to control the amount ofcurrent diverted to flow through a respective alternative current pathso that the voltage across the corresponding main semiconductorswitching element approaches the voltage reference.

Such an arrangement desirably helps to ensure that each of the variousmain semiconductor switching elements has a uniform voltage thereacross,which most preferably is essentially equal to the voltage across each ofthe other main semiconductor switching elements.

In a preferred embodiment of the invention the or each control unit isconfigured to control the amount of current diverted to flow through arespective alternative current path by selectively switching thecorresponding auxiliary semiconductor switching element on and off.

Such control is desirable so as not to create a further problem whileattempting to alleviate the difficulties associated with differingturn-off performance characteristics between main semiconductorswitching elements.

In another preferred embodiment of the invention the or each controlunit is further configured to switch the said corresponding auxiliarysemiconductor switching element on and off within a switching operationa plurality of times in an operating cycle of the semiconductorswitching string.

Carrying out an on/off switching operation a plurality of times within agiven operating cycle of the semiconductor switching string, i.e. whileeach of the main semiconductor switching elements within the string isin either a reverse-biased or a forward-biased condition, helps toensure that the level of current flowing through a respectivealternative current path, and hence the level of current flowing throughthe corresponding auxiliary semiconductor switching element, remains ata level required to compensate for the aforementioned variation inturn-off performance characteristics.

Optionally the ratio of time for which a said corresponding auxiliarysemiconductor switching element is on and off within a switchingoperation differs during the operating cycle.

Such a switching regime allows the semiconductor switching string of theinvention to adapt to a change in, e.g. the stored charge remaining in agiven main semiconductor switching element.

Preferably each semiconductor switching assembly includes an auxiliarysemiconductor switching element connected in inverse-parallel with thecorresponding main semiconductor switching element whereby when theinverse-parallel connected auxiliary semiconductor switching element isswitched on the alternative current path is configured to allow currentto flow from the second connection terminal to the first connectionterminal of the corresponding main semiconductor switching element, andwherein the corresponding control unit is configured to switch on arespective inverse-parallel connected auxiliary semiconductor switchingelement while the corresponding main semiconductor switching element isin a reverse-biased condition to divert current to flow through theso-configured alternative current path to reduce the voltage across thecorresponding main semiconductor switching element.

Having each auxiliary semiconductor switching element arranged in such amanner, and the corresponding control unit configured in such a manner,allows the semiconductor switching string of the invention to compensatefor the greatest impact of a variation in the turn-off performancecharacteristics of a string of series-connected main semiconductorswitching elements, namely the resulting variation in reverse recoveredcharge stored by each main semiconductor switching element which ariseswhen the main semiconductor switching elements continue to conductcurrent for differing periods of time when in the reverse-biasedcondition after forward conduction has ceased.

Each inverse-parallel connected auxiliary semiconductor switchingelement may be or include a transistor having an emitter connected tothe first connection terminal of the corresponding main semiconductorswitching element, a collector connected to the second connectionterminal of the corresponding main semiconductor switching element, anda base connected to the corresponding control unit.

A transistor, especially one incorporating a wide-band-gapsemiconducting material such as silicon carbide, gallium nitride ordiamond, has the required high voltage performance characteristicnecessary to match or even exceed that of the corresponding mainsemiconductor switching element, while at the same time permitting thepassage therethrough of a relatively small amount of current necessaryto affect the desired reduction in voltage across the corresponding mainsemiconductor switching element.

In a still further preferred embodiment of the invention eachsemiconductor switching assembly includes an auxiliary semiconductorswitching element connected in parallel with the corresponding mainsemiconductor switching element whereby when the parallel connectedauxiliary semiconductor switching element is switched on the alternativecurrent path is configured to allow current to flow from the firstconnection terminal to the second connection terminal of thecorresponding main semiconductor switching element, and wherein thecorresponding control unit is configured to switch on a respectiveparallel connected auxiliary semiconductor switching element while thecorresponding main semiconductor switching element is in aforward-biased condition to divert current to flow through theso-configured alternative current path.

Such an arrangement desirably allows the semiconductor switching stringof the invention to compensate for different performance characteristicsof respective main semiconductor switching elements and inherentlimitations in their performance which are manifest when the mainsemiconductor switching elements are in the forward-biased condition,i.e. each main semiconductor switching element is switched off butexperiences a positive voltage between its first and second connectionterminals such that it will allow current to flow therethrough onreceipt of a turn-on signal from the corresponding control unit.

Optionally each parallel connected auxiliary semiconductor switchingelement is or includes a transistor having an emitter connected to thesecond connection terminal of the corresponding main semiconductorswitching element, a collector connected to the first connectionterminal of the corresponding main semiconductor switching element, anda base connected to the corresponding control unit.

As mentioned above, transistors, especially those incorporatingwide-band-gap to semiconducting materials, have desirable high voltageperformance characteristics.

In another embodiment of the invention each semiconductor switchingassembly includes a first auxiliary semiconductor switching elementconnected in inverse-parallel with the corresponding main semiconductorswitching element and a second auxiliary semiconductor switching elementconnected in parallel with the corresponding main semiconductorswitching element.

Such an arrangement provides each semiconductor switching assembly withbi-directional functionality whereby it is able to selectively reducethe voltage across a corresponding main semiconductor switching elementwhen the said main semiconductor switching element is eitherreverse-biased or forward-biased so as to permit grading, i.e. balancingof the respective voltages across the various main semiconductorswitching elements, when the various main semiconductor switchingelements are in either such biased conditions.

Optionally each semiconductor switching assembly includes an auxiliarysemiconductor switching element selectively connectable ininverse-parallel and in parallel with the corresponding mainsemiconductor switching element whereby when the auxiliary semiconductorswitching element is connected in inverse-parallel and switched on afirst alternative current path allows current to flow from the secondconnection terminal to the first connection terminal of thecorresponding main semiconductor switching element, and whereby when theauxiliary semiconductor switching element is connected in parallel andswitched on a second alternative current path allows current to flowfrom the first connection terminal to the second connection terminal ofthe corresponding main semiconductor switching element.

The inclusion in each semiconductor switching assembly of an auxiliarysemiconductor switching element which is selectively connectable in bothinverse-parallel and parallel with the corresponding main semiconductorswitching element provides the ability to grade the voltages across themain semiconductor switching elements when they are eitherreverse-biased or forward-biased using only one auxiliary semiconductorswitching element.

Each auxiliary semiconductor switching element may be connected withfirst and second pairs of passive current check elements in a fullbridge arrangement between the first and second connection terminals ofthe corresponding main semiconductor switching element.

Such an arrangement permits current to flow through a respectiveauxiliary semiconductor switching element when flowing both from thesecond connection terminal of the corresponding main auxiliary switchingelement to the first connection terminal thereof and vice versa.

There now follows a brief description of preferred embodiments of theinvention, by way of non-limiting example, with reference to theaccompanying drawings in which:

FIG. 1(a) shows a schematic view of a first semiconductor switchingassembly which forms a part of a semiconductor switching stringaccording to a first embodiment of the invention;

FIG. 1(b) shows a schematic view of two of the first semiconductorswitching assemblies shown in FIG. 1(a) connected in series with oneanother to form a portion of the semiconductor switching stringaccording to the first embodiment of the invention;

FIG. 2(a) illustrates how a difference in the turn-off performancecharacteristics of respective main semiconductor switching elements ineach of the semiconductor switching assemblies shown in FIG. 1(b) iscompensated for;

FIG. 2(b) illustrates the resulting change in voltage across each of themain semiconductor switching elements as the compensation shown in FIG.2(a) takes place;

FIG. 3 shows a schematic view of a second semiconductor switchingassembly which forms a part of a semiconductor switching stringaccording to a second embodiment of the invention; and

FIG. 4 shows a schematic view of a fourth semiconductor switchingassembly which forms a part of a semiconductor switching stringaccording to a fourth embodiment of the invention.

The first semiconductor switching assembly 10 includes a mainsemiconductor switching element 12 which has first and second connectionterminals 14, 16. In the embodiment shown the main semiconductorswitching element 12 is a main thyristor 18, although in otherembodiments of the invention a different semiconductor switching elementmay be used such as a diode, Light-Triggered Thyristor (LTT), GateTurn-Off thyristor (GTO), Gate Commutated Thyristor (GCT) or IntegratedGate Commutated Thyristor (IGCT). Preferably the main semiconductorswitching element 12 is optimised for lowest conduction (on-state)losses at the expense of other parameters such as turn-on and turn-offcharacteristics and off-state dv/dt capability.

The main thyristor 18 shown includes an anode 20 which defines the firstconnection terminal 14, a cathode 22 which defines the second connectionterminal 16, and a gate 24 that defines a control terminal 26 via whichthe main thyristor 18 may be switched on.

When the main thyristor 18 is so switched on, i.e. turned-on fully,current flows through the main thyristor 18 from the first connectionterminal 14 to the second connection terminal 16, i.e. from the anode 20to the cathode 22.

The main thyristor 18 has an auxiliary semiconductor switching element28 which is electrically connected between the first and secondconnection terminals 14, 16 of the main thyristor 18, and the auxiliarysemiconductor switching element 28 has a control unit 30 that isoperatively connected therewith. The control unit 30 is configured toswitch on the auxiliary semiconductor switching element 28 toselectively create an alternative current path 32 between the first andsecond connection terminals 14, 16.

More particularly the auxiliary semiconductor switching element 28 isconnected in inverse-parallel with the main thyristor 18 such that whenthe auxiliary semiconductor switching element is switched on theresulting alternative current path 32 is configured to allow current toflow from the second connection terminal 16 to the first connectionterminal 14.

More particularly still the auxiliary semiconductor switching element 28includes a transistor 34 which has an emitter 36 that is connected tothe first connection terminal 14 of the main thyristor 18, a collector38 that is connected to the second connection terminal 16 of the mainthyristor 18, and a base 40 that is connected to the control unit 30.

The transistor 34 shown in FIG. 1 is an n-channel insulated-gate bipolartransistor (IGBT), although many other transistors may also be used suchas, for example, a bipolar junction transistor (BJT), ametal-oxide-semiconductor field-effect transistor (MOSFET), or ajunction gate field-effect transistor (JFET). A transistor assembly,such as a MOSFET-JFET cascode circuit incorporating a super-cascodearrangement of 50V MOSFETs and a series string of 1200V SiC JFETs, or adirect series connection of low voltage MOSFETs or IGBTs, may also beused. In any event the transistor 34 shown has a relatively high voltagerating of approximately 9 kV to 10 kV, but a relatively low currentrating of only a few tens of amps.

In the embodiment shown the transistor 34 has an anti-parallel diode 42connected thereacross which protects the transistor 34 from reversevoltages while the main thyristor 18 is forward-biased. In otherembodiments of the invention (not shown) the separate anti-paralleldiode 42 could be omitted and instead use made of an intrinsicbody-diode which is included within some transistors.

The auxiliary semiconductor switching element 28 shown in FIG. 1(a) alsoincludes an optional current limiting element 44, in the form of aresistor 46, which is connected in series with the transistor 34 andanti-parallel diode 42 combination mentioned above. The auxiliarysemiconductor switching element 28 also additionally includes a further,series-connected diode 48 which is arranged to permit the flow ofcurrent through the alternative current path 32 in the same direction asthrough the transistor 34. The further, series-connected diode 48 isincluded, in conjunction with the anti-parallel diode 42, to protect thetransistor 34 from reverse voltages while the main thyristor 18 isforward-biased.

In other embodiments of the invention in which the auxiliarysemiconductor switching element 28, i.e. the transistor 34, is capableof withstanding reverse voltage (while the main semiconductor switchingelement 12, i.e. the main thyristor 18, is forward-biased) theanti-parallel diode 42 and the series-connected diode 48 may be omitted.

As well as having the auxiliary semiconductor switching element 28connected in inverse-parallel therewith, the main thyristor 18 also hasa damping circuit (which includes a damping capacitor 50 and a dampingresistor 52), as well as a further resistor 54, i.e. a DC gradingresistor, connected in parallel between the first and second connectionterminals 14, 16.

In use an ideal thyristor would cease to conduct exactly at the instantwhen the current flowing through the thyristor falls to zero. However areal thyristor, such as the main thyristor 18 shown in FIG. 1, continuesto conduct current in a reverse direction (even when the main thyristor18 is switched off and in a so-called reverse-biased condition) for somehundreds of microseconds after the current falls to zero, as illustratedschematically in FIG. 2(a). The time integral of this reverse current isthe ‘reverse recovered charge’ (Q,), i.e. stored charge, of the mainthyristor 18.

In the embodiment shown, the main thyristor 18 has a lower Q_(rr)than,e.g. a second main thyristor 56 in an otherwise identical further firstsemiconductor switching assembly 10 which is connected in series withthe first semiconductor switching assembly 10 that includes the firstmain thyristor 18, as shown in FIG. 1(b).

In this way the two first semiconductor switching assemblies 10 togetherdefine a portion of the semiconductor switching string 100 according tothe first embodiment of the invention, which additionally includesfurther series-connected first semiconductor switching assemblies 10(not shown). The two semiconductor switching assemblies 10 shown eachhas its own corresponding control unit 30. In other embodiments,however, one or more such semiconductor switching assemblies 10 within agiven semiconductor switching string may share a common control unit.

Meanwhile, the aforementioned difference in Q, between the first andsecond main thyristors 18, 56 arises because the first main thyristor 18starts to turn off sooner than the second main thyristor 56. As a resultthe reverse current flowing though the first main thyristor 18 willstart to reduce sooner than in the second main thyristor 56, as alsoshown in FIG. 2(a).

When the first and second main thyristors 18, 56 are connected in theseries arrangement shown in FIG. 1(b), i.e. as a portion of the firstsemiconductor switching string 100, the current flowing through thefirst semiconductor switching assembly 10 (i.e. the switching assemblyincluding the first main thyristor 18) must be the same as the currentflowing through the further first semiconductor switching assembly 10(i.e. the switching assembly including the second main thyristor 56).Since the first main thyristor 18 turns off sooner (and so no longerconducts current) the difference in reverse current flows into thedamping circuit, i.e. the damping capacitor 50 and the damping resistor52, of the first main thyristor 18. This causes the voltage V across thefirst main thyristor 18 to build up sooner, and reach a larger reversepeak voltage (as shown by a first dashed line 18 in FIG. 2(b)), than thesecond main thyristor 56 with a higher Q_(rr) (as shown by a seconddashed line 56 in FIG. 2(b)).

Such operation, if left un-checked, gives rise to a voltage offset ΔVbetween the voltage across the first main thyristor 18 and the voltageacross the second main thyristor 56, where the voltage offset ΔV isgiven by:ΔV=ΔQ _(rr) /C _(d)where

-   -   ΔQ_(rr) is the difference in charge stored by the second main        thyristor 56 and the first main thyristor 18, and    -   C_(d) is the value of the damping capacitor 50.

Such a voltage offset can persist for a long time such that it does notdecay significantly before the first main thyristor 18 is turned onagain approximately 240 electrical degrees later. Such a voltage offsetcan also significantly affect the timing point at which the voltageacross a given main thyristor 18, 56 crosses zero. This impacts on theaccuracy of an extinction angle that must be established, e.g. when themain thyristors 18, 56 form part of a HVDC power converter which isoperating as an inverter and requires that the extinction angle includesa margin to accommodate such variations in stored charge.

However, in the case of the first semiconductor switching string 100 ofthe invention, each control unit 30 is configured to switch on thecorresponding auxiliary semiconductor switching element 28, i.e. thecorresponding transistor 34, while the corresponding first mainthyristor 18 is in the aforementioned reverse-biased condition and whilea reverse current I is flowing through the said first main thyristor 18,to create the corresponding alternative current path 32 and therebydivert the reverse current through the corresponding alternative currentpath 32. Such diversion of the reverse current through the correspondingalternative current path 32 prevents this current flowing into theassociated damping circuit which has the effect of inhibiting the buildup of voltage across the first main thyristor 18 (and so is equivalentto reducing the effective off-state impedance of the corresponding firstmain thyristor 18) such that the resulting voltage across thecorresponding first main thyristor 18 is reduced.

More particularly, each control unit 30 is configured to control theamount of current directed to flow through the corresponding alternativecurrent path 32 by switching the corresponding transistor 34 on and offwithin a particular switching operation s₁, s₂, s₃, s₄, 55, and to carryout such a switching operation s₁, s₂, s₃, s₄, s₅ five times during agiven operating cycle of the semiconductor switching string 100, i.e.while each main semiconductor switching element 12, i.e. the mainthyristors 18, 56, is in the reverse-biased condition. As shown in FIG.2(a), the ratio of time for which the transistor 34 is on (shown asshaded in FIG. 2(a)) and off within each switching operation s₁, s₂, s₃,s₄, s₅ is different in each switching operation s₁, s₂, s₃, s₄, s₅ so asto adapt the switching of the transistor 34 to the changing differencein the voltage across the first and second main thyristors 18, 56. Inother embodiments of the invention (not shown) more than or fewer thanfive switching operations may be included in a given operating cycle ofthe semiconductor switching string 100, and the on/off ratio during eachsuch switching operation may further differ from those shown in FIG.2(a).

In addition each control unit 30 is also configured to switch on thecorresponding transistor 34 when the voltage across the correspondingfirst main thyristor 18 differs from a voltage reference derived fromthe voltage across all of the main semiconductor switching elements 12in the string 100.

In the first embodiment of semiconductor switching string 100 thevoltage reference is equivalent to the average voltage across all of themain semiconductor switching elements 12 in the string 100, i.e. theaverage voltage across the first and second main thyristors 18, 56 shownand all of the remaining main thyristors (not shown). Such an averagevoltage could be established by a high bandwidth voltage divider acrossseries-connected first and second main thyristors 18, 56, or by havingeach main thyristor 18, 56 report its own voltage back to itscorresponding control unit 30 (or another, overarching control system orhigher level controller) and for one of the control units 30 (or theoverarching control system) to compute the average voltage (and, in thecase of an overarching control system, have that system re-transmit asignal representing the said computed average voltage to each of thecontrol units 30).

By way of example, with reference to the portion of the firstsemiconductor switching string 100 shown in FIG. 1(b), the control unit30 of the first semiconductor switching assembly 10 compares the voltageacross the first main thyristor 18 with the average voltage and, whenthe voltage across the first main thyristor 18 is higher than theaverage voltage, the control unit 30 switches the transistor 34 of theinverse-parallel connected auxiliary semiconductor switching element 28on and off during the five switching operations s₁, s₂, s₃, s₄, s₅ so asto selectively direct current through the alternative current path 32and thereby selectively inhibit the build up of voltage across the firstmain thyristor 18. Such switching has the effect of passing an amount ofcharge between the first and second terminals 14, 16 of the first mainthyristor 18 which is equal to the difference in the Q_(rr) of the firstmain thyristor 18 and the Q_(rr) of the second main thyristor 56.

As indicated, this has the effect of reducing the voltage across thefirst main thyristor 18 (as illustrated by a first non-dashed line 18′in FIG. 2(a)) while increasing the voltage across the second mainthyristor 56, which was below the average voltage (as illustrated by asecond non-dashed line 56′ in FIG. 2(a)). This has the net effect thatthe voltage across both main thyristors 18, 56 is brought closer to theaverage voltage, i.e. the voltage across both main thyristors 18, 56approaches the voltage reference, such that following a number ofswitching operations, e.g. five in the illustrated example, thevariation in the voltage across the main thyristors 18, 56 (and hencethe variation in Q_(rr) between the main thyristors 18, 56 arising fromtheir differing turn-off performance characteristics) is compensated forand cancelled out.

Such compensation for the variation in Q_(rr) between the mainthyristors 18, 56 has the additional benefit of reducing by between 70%and 90% the required capacitance C_(d) of the damping capacitor 50 andthe power rating of the damping resistor 52.

In other embodiments of the invention (not shown) the or each controlunit 30 may instead utilise known or measured characteristics of thecorresponding main semiconductor switching element 12 to formulate aswitching regime for the corresponding inverse-parallel connectedauxiliary semiconductor switching element 28. Such a switching regimemay be adapted to the said characteristics of the main semiconductorswitching element 12, e.g. the or each control unit 30 ‘learns’ thebehaviour of the corresponding main semiconductor switching element 12and modifies the switching of the corresponding inverse-parallelconnected auxiliary semiconductor switching element 28 accordingly.

In the foregoing manner, within the semiconductor switching string 100of series-connected first semiconductor switching assemblies 10, eachcontrol unit 30 (or a single control unit operatively connected to theinverse-parallel connected auxiliary semiconductor switching element 28of each main thyristor 18, if so configured) compares the voltage acrossits corresponding main thyristor 18 with the voltage reference.

Those main thyristors 18 whose voltage is above the voltage referencethen have their inverse-parallel connected auxiliary semiconductorswitching element 28 switched on and off by the corresponding controlunit 30 to selectively divert current through the correspondingalternative current path 32 in order to reduce the voltage across eachof the said above-average voltage main thyristors 18, so as to bring thevoltage of these main thyristors 18 closer to the voltage reference.This has the effect of increasing the average voltage of the remainingbelow-average voltage main thyristors until eventually, i.e. after anumber of similar switching operations, all the main thyristors exceptthe one with the longest turn-off time (i.e. the largest Q_(rr)) havehad their respective turn-off time discrepancies (i.e. their respectiveQ_(rr) discrepancies) compensated for.

A second semiconductor switching assembly, which is one of a pluralityof series-connected such semiconductor switching assemblies thattogether form a semiconductor switching string according to a secondembodiment of the invention, is shown in FIG. 3 and is designatedgenerally by reference numeral 70.

The second semiconductor switching assembly 70 is similar to the firstsemiconductor switching assembly 10 and like features share the samereference numerals.

However, the second semiconductor switching assembly 70 differs from thefirst semiconductor switching assembly 10 in that it includes anauxiliary semiconductor switching element 72 that is connected inparallel with the main semiconductor switching element 12, i.e. the mainthyristor 18, such that when the parallel connected auxiliarysemiconductor switching element 72 is switched on the resultingalternative current path 32 allows current to flow from the firstconnection terminal 14 to the second connection terminal 16 of the mainthyristor 18.

A further difference is that in the second semiconductor switchingassembly 70 the control unit 30 is configured to switch on the parallelconnected auxiliary semiconductor switching element 72 while the mainthyristor 18 is in a forward-biased condition, i.e. when the mainthyristor 18 is switched off but experiences a positive voltage betweenits first and second connection terminals 14, 16 such that it will allowcurrent to flow through the main thyristor 18 in a normal manner fromthe anode 20 to the cathode 22 on receipt of a turn-on signal from thecontrol unit 30.

The parallel connected auxiliary semiconductor switching element 72similarly includes a transistor 34 but, because of the parallel mannerin which the auxiliary semiconductor switching element 72 iselectrically connected with the main thyristor 18, the emitter 36 of thetransistor 34 is instead connected to the second connection terminal 16of the main thyristor 18 and the collector 38 of the transistor 34 isinstead connected to the first connection terminal 14 of the mainthyristor 18. The base 40 of the transistor 34 is again connected to thecontrol unit 30.

The transistor 34 in the parallel connected auxiliary semiconductorswitching element 72 has the same ratings as the transistor 34 in theinverse-parallel connected auxiliary semiconductor switching element 28of the first semiconductor switching assembly 10, i.e. a relatively highvoltage rating of approximately 9 kV to 10 kV and a relatively lowcurrent rating of only a few tens of amps.

The second semiconductor switching assembly 70 may be operated inessentially the same manner as the first semiconductor switchingassembly 10 in order to selectively divert current through thealternative current path 32 (to reduce the effective impedance of themain thyristor 18 and thereby reduce the voltage across the mainthyristor 18) while the main thyristor 18 is in a forward-biasedcondition. In other words, the control unit 30 of the secondsemiconductor switching assembly 70 is configured to switch the parallelconnected auxiliary semiconductor switching element 72 on and off withina plurality of switching operations to control the amount of currentdiverted to flow through the alternative current path 32 (and hence tocontrol the amount of charge passed between the first and secondterminals 14, 16 of the main thyristor 18) so that the voltage acrossthe main thyristor 18 approaches a voltage reference.

As such the semiconductor switching string according to a secondembodiment of the invention, i.e. including a plurality ofseries-connection second semiconductor switching assemblies 70, isoperable in a similar manner to the first semiconductor switching string100 to permit grading, i.e. balancing, of the voltage across theplurality of respective main semiconductor switching elements 12 whileeach such main semiconductor switching element 12 is in a forward-biasedcondition.

A still further semiconductor switching string according to a thirdembodiment of the invention (not shown) includes a plurality ofseries-connected third semiconductor switching assemblies, each of whichincludes both an inverse-parallel connected auxiliary semiconductorswitching element 28 (as per the first semiconductor switching assembly10) and a parallel connected auxiliary semiconductor switching element72 (as per the second semiconductor switching assembly 70).

In this regard, despite the use of a control unit 30 to switch theinverse-parallel connected auxiliary semiconductor switching element 28on and off while the main thyristor 18 is in a reverse-biased conditionin order to compensate for any variation in Q_(rr) between respectivemain thyristors 18, 56 (and the voltage imbalance across the mainthyristors 18, 56 that would otherwise arise), residual voltageunbalancing effects may arise while the main thyristor 18 remainsswitched off but is forward biased, i.e. during the period of delaybetween the main thyristor 18 being switched on and it becoming fullyturned on, i.e. during turn on of the main thyristor 18.

Such residual voltage unbalancing effects may, for example, arisebecause of one or more unintended variations in the level of reversecurrent diverted through a respective alternative current path 32provided by the corresponding inverse-parallel connected auxiliarysemiconductor switching element 28 when reducing the voltage across thecorresponding main thyristor 18 while it is reverse-biased.

In addition, in instances when the main thyristor 18 is self-commutated,voltage imbalances can arise at turn-off of such main thyristors 18because of differences between respective main thyristors 18 in the timetaken for the forward current after turn-off to decay to zero, which isessentially equivalent to dissipating a given amount of stored charge.

The third semiconductor switching assembly, and its inclusion of bothinverse-parallel and parallel connected auxiliary semiconductorswitching elements 28, 72, is able to compensate for any variation inQ_(rr) between respective main thyristors 18, 56 (and achieve a balancein the voltage across the respective main thyristors 18, 56) when themain thyristors 18, 56 are both reverse-biased and forward-biased. Assuch the third semiconductor switching assembly permits a completeomission of the damping circuit, i.e. the damping capacitor 50 and thedamping resistor 52, while still permitting the third semiconductorswitching string of which it forms a part to carry out voltage gradingin respect of each of the main semiconductor switching elements 12therein.

A fourth semiconductor switching assembly, which is one of a pluralityof series-connected such semiconductor switching assemblies thattogether form a semiconductor switching string according to a fourthembodiment of the invention, is shown in FIG. 4 and is designatedgenerally by reference numeral 110.

The fourth semiconductor switching assembly 110 has similarities witheach of the first, second and third semiconductor switching assemblies10; 70 and like features share the same reference numerals.

The fourth semiconductor switching assembly 110 is, however, differentfrom each of the aforementioned semiconductor switching assemblies 10;70 in that it includes an auxiliary semiconductor switching element 112that is selectively connectable in both inverse-parallel and parallelwith a corresponding main semiconductor switching element 12 such thatthe auxiliary semiconductor switching element 112 has bidirectionalfunctionality. In this regard, when the bidirectional auxiliarysemiconductor switching element 112 is connected in inverse-parallelwith the main semiconductor switching element 112 (and switched on) afirst alternative current flow path 114 allows current to flow from thesecond connection terminal 16 to the first connection terminal 14 of themain semiconductor switching element 12. Meanwhile, when thebidirectional auxiliary semiconductor switching element 112 is connectedin parallel with the main semiconductor switching element 112 (andswitched on) a second alternative current flow path 116 allows currentto flow from the first connection terminal 14 to the second connectionterminal 16 of the main semiconductor switching element 12.

More particularly, such bidirectional functionality is provided byconnecting the bidirectional auxiliary semiconductor switching element112 with first and second pairs 118, 120 of passive current checkelements 122, i.e. devices which allow current to flow in one directiononly which in the embodiment shown are diodes 124, in a full bridgearrangement between the first and second connection terminals 14, 16 ofthe main semiconductor switching element 12.

The fourth semiconductor switching assembly 110 operates in a similarmanner to the third semiconductor switching assembly, in that itsinclusion of a bidirectional auxiliary semiconductor switching element112 which is selectively connectable in either inverse-parallel orparallel with the main semiconductor switching element 12 allows it tocompensate for any variation in Q_(rr) between its main semiconductorswitching element 12, i.e. main thyristor 18, and the other mainsemiconductor switching elements 12 in the fourth semiconductorswitching string of which it forms a part (and thereby grade the voltageacross the said various main semiconductor switching elements 12 in thefourth string) when the main semiconductor switching elements 12 areeither reverse-biased or forward-biased.

As such the fourth semiconductor switching assembly 110 permits acomplete omission of the damping circuit, i.e. the damping capacitor 50and the damping resistor 52, while still permitting the fourthsemiconductor switching string of which it forms a part to carry out theaforementioned voltage grading in respect of each of the mainsemiconductor switching elements 12 therein.

In addition to the foregoing, a semiconductor switching string (notshown) according to a further embodiment of the invention may include aplurality of series-connected semiconductor switching assemblies made upof any combination of the first, second, third or fourth semiconductorswitching assemblies 10; 70; 110; described hereinabove.

The invention claimed is:
 1. A semiconductor switching string, for usein a HVDC power converter, comprising: a plurality of series-connectedsemiconductor switching assemblies, each semiconductor switchingassembly having a main semiconductor switching element including firstand second connection terminals between which current flows from thefirst terminal to the second terminal when the main semiconductorswitching element is switched on, the main semiconductor switchingelement having an auxiliary semiconductor switching element electricallyconnected between the first and second connection terminals thereof; anda control unit operatively connected with each auxiliary semiconductorswitching element, the or each control unit being configured to switchon a respective auxiliary semiconductor switching element to selectivelycreate an alternative current path between the first and secondconnection terminals associated therewith whereby current is diverted toflow through the alternative current path to reduce the voltage acrossthe corresponding main semiconductor switching element, the or eachcontrol unit being further configured to switch on the said respectiveauxiliary semiconductor switching element when the voltage across thecorresponding main semiconductor switching element differs from areal-time varying voltage reference equivalent to the average voltageacross all of the main semiconductor switching elements in thesemiconductor switching string, and the or each control unit being stillfurther configured to control the amount of current diverted to flowthrough a respective alternative current path so that the voltage acrossthe corresponding main semiconductor switching element approaches thereal-time varying voltage reference.
 2. A semiconductor switching stringaccording to claim 1 wherein the or each control unit is configured toswitch on a said respective auxiliary semiconductor switching elementwhen the voltage across the corresponding main semiconductor switchingelement is greater than the voltage reference.
 3. A semiconductorswitching string according to claim 1 wherein the or each control unitis configured to control the amount of current diverted to flow througha respective alternative current path by selectively switching thecorresponding auxiliary semiconductor switching element on and off.
 4. Asemiconductor switching string according to claim 3 wherein the or eachcontrol unit is further configured to switch the said correspondingauxiliary semiconductor switching element on and off within a switchingoperation a plurality of times in an operating cycle of thesemiconductor switching string.
 5. A semiconductor switching stringaccording to claim 3 wherein the ratio of time for which a saidcorresponding auxiliary semiconductor switching element is on and offwithin a switching operation differs during the operating cycle.
 6. Asemiconductor switching string according to claim 1 wherein eachsemiconductor switching assembly includes an auxiliary semiconductorswitching element connected in inverse-parallel with the correspondingmain semiconductor switching element whereby when the inverse-parallelconnected auxiliary semiconductor switching element is switched on thealternative current path is configured to allow current to flow from thesecond connection terminal to the first connection terminal of thecorresponding main semiconductor switching element, and wherein thecorresponding control unit is configured to switch on a respectiveinverse-parallel connected auxiliary semiconductor switching elementwhile the corresponding main semiconductor switching element is in areverse-biased condition to divert current to flow through theso-configured alternative current path to reduce the voltage across thecorresponding main semiconductor switching element.
 7. A semiconductorswitching string according to claim 6 wherein each inverse-parallelconnected auxiliary semiconductor switching element is or includes atransistor having an emitter connected to the first connection terminalof the corresponding main semiconductor switching element, a collectorconnected to the second connection terminal of the corresponding mainsemiconductor switching element, and a base connected to thecorresponding control unit.
 8. A semiconductor switching stringaccording to claim 1 wherein each semiconductor switching assemblyincludes an auxiliary semiconductor switching element connected inparallel with the corresponding main semiconductor switching elementwhereby when the parallel connected auxiliary semiconductor switchingelement is switched on the alternative current path is configured toallow current to flow from the first connection terminal to the secondconnection terminal of the corresponding main semiconductor switchingelement, and wherein the corresponding control unit is configured toswitch on a respective parallel connected auxiliary semiconductorswitching element while the corresponding main semiconductor switchingelement is in a forward-biased condition to divert current to flowthrough the so-configured alternative current path.
 9. A semiconductorswitching string according to claim 8 wherein each parallel connectedauxiliary semiconductor switching element is or includes a transistorhaving an emitter connected to the second connection terminal of thecorresponding main semiconductor switching element, a collectorconnected to the first connection terminal of the corresponding mainsemiconductor switching element, and a base connected to thecorresponding control unit.
 10. A semiconductor switching stringaccording to claims 1 wherein each semiconductor switching assemblyincludes a first auxiliary semiconductor switching element connected ininverse-parallel with the corresponding main semiconductor switchingelement and a second auxiliary semiconductor switching element connectedin parallel with the corresponding main semiconductor switching element.11. A semiconductor switching string according to claim 1 wherein eachsemiconductor switching assembly includes an auxiliary semiconductorswitching element selectively connectable in inverse-parallel and inparallel with the corresponding main semiconductor switching elementwhereby when the auxiliary semiconductor switching element is connectedin inverse-parallel and switched on a first alternative current pathallows current to flow from the second connection terminal to the firstconnection terminal of the corresponding main semiconductor switchingelement, and whereby when the auxiliary semiconductor switching elementis connected in parallel and switched on a second alternative currentpath allows current to flow from the first connection terminal to thesecond connection terminal of the corresponding main semiconductorswitching element.
 12. A semiconductor switching string according toclaim 11 wherein each auxiliary semiconductor switching element isconnected with first and second pairs of passive current check elementsin a full bridge arrangement between the first and second connectionterminals of the corresponding main semiconductor switching element.